Production of aluminum and aluminum alloys



April 28, 1970 G. L. HERWIG ETAL 5 1 PRODUCTION OF ALUMINUM AND ALUMINUMALLOYS Filed Sept. 6, 1966 zz 241920 16 u 245164 14213 KP-Pw I UnitedStates Patent Ofitice 3,508,908 PRODUCTION OF ALUMINUM AND ALUMINUMALLOYS George Lang Herwig, East Brighton, Victoria, and Ernest Foley,East Burwood, Victoria, Australia, assign-ors to Conzinc Riotinto ofAustralia Limited, Melbourne, a corporation of Victoria, andCommonwealth Scientific and Industrial Research Organization, Victoria,Australia, a body corporate of Australia Filed Sept. 6, 1966, Ser. No.577,473 Claims priority, application Australia, Sept. 8, 1965,

63,759/65; Aug. 29, 1966, 10,290/66 Int. Cl. C22b 21/02; C22d 3/12 U.S.Cl. 75-68 Claims ABSTRACT OF THE DISCLOSURE A process for the productionof aluminum and aluminum alloys wherein a fused salt electrolytecontaining at least one alkali metal halide is electrolyzed using acathode of a molten metal or alloy, in which the electrolyte comprises ahalde of a metallic reductant for aluminum chloride which reductant issoluble in the cathode, the metallic reductant being deposited byelectrolysis from the electrolyte and dissolved in the cathode, and thecathode metal or alloy and the contained reductant being brought intocontact with a double salt of alchlor.

This invention relates to the production of aluminum and aluminum alloysfrom aluminum chloride (hereinafter referred to as alchlor) or fromsalts containing alchlor, and refers especially to a process of andapparatus for reducing a salt containing alchlor to aluminum by reactionwith a metal reductant which may, for example, be magnesium. Theinvention also includes a process and apparatus for the production ofaluminum by fusion electrolysis using alchlor as the feed material.

The alchlor is preferably used in the form of a double salt, for examplea low melting point double salt containing sodium chloride and alchlor.

It will be appreciated that, depending on whether the amount of alchlorpresent in the salt phase is equal to or is greater or less than thestoichiometric proportion, there will be present either a true doublesalt or a solution of alchlor in a salt. The term double salt is used inthe specification and claims hereof to include either (a) a double saltof alchlor and the halide of an alkali metal or (b) a solution ofalchlor in a salt or salt phase.

The invention broadly resides in a process for the production ofaluminum and aluminum alloys which comprises reacting a metallicreductant with a double salt of alchlor.

In one form of the invention molten magnesum is reacted with a moltendouble salt of sodium chloride and aluminum chloride. Some advantages ofusing the double salt are the ease of handling and the relatively lowvapour pressure of aluminum chloride above the salt at temperatures inthe vicinity of 750 C. (i.e. above the melting point of aluminum).

It is advantageous to add a proportion of the circulating aluminum oraluminum-magnesium alloy to the molten magnesium to ensure that thedensty of the metallic phase at all times is greater than that of themolten saline phase.

The reaction may be carried out in a reactor associated with anelectrolytc cell in which case the magnesium chloride formed by thereaction may be treated electrolytically for recovery of magnesium andchlorine.

Electrolytic processes for the production of aluminum which are inpresent day use employ aluminum oxide as a solution in a double salt ofsodium and aluminum 3,508,908 Patented Apr. 28, 1970 fiuorides.Disadvantages of this system include 1) the Consumption of high puritycarbon electrodes by the oxygen Component of the feed material at a ratewhich varies from 0.6 to 1.0 lb. of carbon per lb. of aluminum produced;(2) the low voltage efi iciency of the cell of 35 to 45% which resultsfrom a summation of high voltage drops across the anode which must beadapted to continuous feeding, across the carbon lining which must bethick to contain the high temperatures within the cell, across the polargap which must be large to allow clearance between the distorted cathodemetal surface and the anode, and finally due to polarisation effects onthe anode as a result of intermittent charging and variation ofelectrolyte compositions; (3) losses of expensive electrolyte byvapourisation occasioned by the open top Construction of the cellnecessitatd by the need to intermittently and evenly feed solid aluminato the electrolyte; (4) severe corrosion and low cell life resultingfrom the high cell temperature of 950 to 1000 C., and (5) an appreciablelabour requrement for introducing feed material.

The art contains records of attempts to produce aluminum from anhydrousalchlor by electrolysis of an electrolyte composed of alchlor dissolvedin certain fused metal halides. A number of disadvantages have preventedcommercial acceptance of such processes. Alchlor is a volatile solidsubliming at about 178 C. Although its vapour pressure may be loweredsubstantally in molten alkali metal chloride baths, alchlor is readilydistilled from such baths when Operating at or about the melting pointof aluminum (661 C.). As a consequence these processes are operated atlow temperatures or under a positive pressure. Eutectic or low meltingpoint fused salt mixtures containing alchlor have very poor electricalconductivity, especially below the melting point of aluminum. At theOperating temperatures of such processes molten solutions of alchlor arecorrosive towards graphite and carbon which are essentially the onlypractical anode materials which can withstand and co-produced chlorine.At low temperatures aluminum is deposited as a solid commingled withelectrolyte which is inconvenient to separate from the metal crystals,and cell voltages at practical current densities are Very high. At hightemperatures cell voltages can be reduced and liquid metal obtained butthe pressurised equipment is expensive and corrosion is severe.

Our invention in one of its aspects seeks to overcome the disadvantagesof processes using electrolytes containing alchlor, while retaining theadvantages of alchlor as a feed material, -by providing aprocess whereinlittle, if any, alchlor can ever be present in the electrolyte.

According to this aspect of the invention a process for the electrolyticproduction of aluminum and aluminum alloys comprises electrolysing afused salt electrolyte containing at least one alkali metal halide usingas cathode a molten metal or alloy, which molten metal or alloy containsin solution a metallic reductant deposted from the electrolyte, andContacting the molten metal or alloy and the contained metallicreductant with a double salt of alchlor or 'with a solution of alchlorin a molten salt.

In addition to alkali metal halides the electrolyte may contain one ormore alkaline earth metal halides and/or rare earth metal halides.

Preferably the alchlor is contacted with the molten metal or alloy insuch a manner that there is negligible alchlor entering the electrolysiscompartment.

More particularly the invention includes a process for the electrolyticproduction of aluminum which comprises electrolysing in an electrolysiscompartment or 'cell a fused salt electrolyte containing an alkali metalhalide, the cathode being molten aluminum or aluminum alloy containing ametallic reductant deposited 'from the electrolyte by the electrolysis,transferring the molten aluminum or aluminum alloy and reductant to areactor compartment or vessel, reacting alchlor with the metallicreductant in the reactor compartment or vessel, transferring thechloride product of the metallic reductant to the electrolysiscompartment or cell, re-cycling molten aluminum containing a lowerconcentration of the metallic reductant back to the electrolysiscompartment or cell, and discharging the molten metal product from thereactor compartment or vessel. f

Again the electrolyte may contain one or more alkaline earth metalhalides and/or rareearth metal halides in additio'to the alkali metalhalide.

According to another aspect of the invention, an impor'tant featureresides in efecti'ng the reacton between t'he'metallc reductant and thealchlor which latter is preferably in the vapour state, in a compartmentor vessel which is distinct or separate from the electrolysiscompartrnent, so that little or no alchlor is permitted to enter theelectrolysis compartnent and consequently the possibility of "its losswith and contarnination of the chlorine produced in the electrolysis,and the attack by the alchlor on the anode material, are substantiallyprevented.

Preferably the reaction between the metallic reductant and the alchloris effected in a two-stage reactor Compartment or vessel, wherein a saltphase capable of absorbing alchlor as a double salt simultaneouslyContacts both the alchlor as vapour and the molten aluminum containing ametallic reductant. In this case a reaction occurs between the reductantand the alchlor double salt which results in a lowering of reductantconcentration in the aluminum. The alumnum from the first stage isre-cycled to the electrolysis compartrnent and the salt phase containingboth the chloride product of the reductant and alchlor to saturation asa double salt or solution is contacted in a second stage to react to orapproaching extinction with fresh molten aluminum and reductant from theelectrolysis compartment.

The aluminum from the second stage passes back to the first stage 'whilethe salt phase containing the chloride product of the reductant but nowdenuded of alchlor is re-cycled to the electrolysis compartment.

In one form of the invention a small portion of the electrolyte may beused as the alchlor-absor-bing salt phase, the portion of electrolytebeing metered at such a rate to the first stage as to ensure that thesalt phase leaving the second stage contains negligible amounts ofalchlor.

Depending upon the degree to which it is desired to reduce the reductantcontent of the metal being re-cycled to the electrolysis compartment orcell two or more stages of contact may be used, in all cases alchlorbeing fed only to the first stage. The two or more stage countercurrentreaction between the reductant electro deposited into the re-cycledaluminum and the alchlor double salt ensures that the salt phase leavingthe reactor which is i re-cycled to the electrolysis compartment isessentially free of alchlor by last being in contact with an excess ofreductant. The aluminum working backto the first stage then still has acontent of reductant suflicient for further reaction in this stage.

In the first stage the salt phase is continuously in contact with thealchlor vapour and absorbs it as fast as the alchlor portion of thedouble salt reacts with the excess reductant. Thus the salt phase leavesthe first stage for the later stages still with a content of alchlor tobe removed by incoming reductant. The quantity of alchlor reactedoverall will be equivalent to the quantity, of reductant removed in thereactor, firstly, because there will be aflquantity of reductant in thefirst stage to react with 4 the alchlor Vapour being fed in, andsecondly because the reductant present in the second stage will reactwith the alchlor (present as a double salt) to or approachingextinction. Thisis ensured -by metering the fresh electrolyte into thefirst stage at a suitable rate when the reductant is an alkaline earthmetal. If an alkal metal is used as reductant it is not necessary topass a portion of the electrolyte through the reactor system as thechloride product forms the necessary double salt with alchlor and thequantity flows are self-adjusting.

When the above quantity requirements are met some excess reductant willexist in the first stage to react with the alchlor in the double saltpresent. Further alchlor vapour can then absorb into the double salt.

Overall the quantity of alchlor vapour absorbed into the double saltmust essentially equal the quantity of reductant reacted out of aluminumwhich is the quantity of reductant produced in the electrolysiscompartment. Therefore for the success-tul operation of the process ofthe invention itis not necessary for the electrolysing current and rateof input of alchlor vapour to be in continuous and exactsynchronisation. The system can only absorb alchlor vapour as fast as itcan react, that is as fast as' reductant isproduced and fed to thereactor, and

it'can be adapted readily to automatic' control of alchlor vapour input.

In the 'case of an alkali metal reductant, alchlor cannot enter theelectrolysis compartment in any appreciable quantity. Alchlor can onlyleave the first stage of the reactor as` a double salt and this can onlybe produced if reductant is reacted to the chloride in this stage. Itfollows that an excess of reductant must always exist in the laterstages which will be capable of reacting essentially all of the alchlorout of the double salt. In the case of'an alkaline earth metal reductantelectrolyte is metered into the first stage to form a double salt Withthe alchlor, the quantity metered being such as to achieve the aboverequirements.

In both cases not only are losses of alchlor via the electrolysiscompartment substantially avoided, but additionally contamination of thehalogen co-product and attack by the alchlor on the anode are eitherprevented or are of negligible degree.

The metallic cathode should comprise a metal or alloy which is moltenwithin the required temperature range, which is a sufiiciently goodsolvent for the metallic'reductant used and which is substantiallyunreactive to alchlor. Aluminum or an alloy of aluminum is preferablyused. A preferred feature of the invention is the employ- 'nent, ascomponents of the fused salt electrolyte, of an alkali metal halide ormixture of alkali metal and alkaline earth metal halides.

The composition of the fused electrolyte should desirably be such 'thatthe metal or metals deposited at the cathde during electrolysis (Le. themetallic reductant) have adequate 'solubility in the' molten metallicca'thode (eg. in molten aluminum or aluminurn alloy),'low solubility inthe saline phase, and, when dissolved "in the molten cathode metal, havesuffic'ient reactivity to reduce alchlor vapour.

As" previously stated',- the metallic reductant is transferred to areactor compartrnent, distinct from the electroIylsis compartmen't,wherein Chemical interaction occurs between the metallic reductant' andalchlor double salt. By this, arrangement it becomes possible to controlthe amount of reductant to any level de'emed necessary to ensure thedesired degree ofiinteraction. Contrary to what might-:be expected .wehave found that alchlor double vsalts Will ,reduce the magnesium contentof an` aluminum alkaline earth metals (eg. calcum) and lithium as themetallic reductant,'and the use of other alkaline earth metal halides oralkali metal halides, and in particular mixtures thereof, as Componentsof the fused salt electrolyte.

Magnesium chloride, or other alkali earth metals, are used in admixturewith the halides of sodum, potassium or other alkali metals. -As asource of both reductant and complexing agent, alkali chlorides may beused alone or in admixture 'with each other.

Other alkali metal or alkaline earth metal halides, specfically thechlorides and/or fluorides, may be added, within the limitations set outabove; to modify and improve the properties of the electrolyte.

In the preferred process of the invention a molten salt comprisingeither;

(a) magnesium chloride admixed -with halides of sodum and/ or potassium,or

(b) lithium chloride alone or admixed with halides of sodum and/ orpotassium,

is electrolysed in an electrolysis compartment using a dependentgraphite or carbon anode and a molten aluminum cathode, the relativedensities of the liquid phases being such that the molten cathoderemains as a discrete phase at the bottom of the electrolytic cell.Under such conditions substantially pure chlorine is evolved from theanode and corresponding amounts of reductant dissolve in the aluminumcathode.

The aluminum plus reductant metal formed is transferred to a separatesecond compartment where it is reacted countercurrently with alchlordouble salt under conditions of high, but not necessarily 100%efficiency. Aluminum or aluminum alloy denuded in reductant is collectedin suitable collection equipment from which it is either recycled to theelectrolysis compartment to maintain the cathode metal at the desiredlevel or tapped off, as required.

The temperature at which the electrolysis and reduction processes arecarried out is desirably maintained at lO-100 C. above the melting pointof the electrolyte, or the cathode metal or the salt phase -followingthe reducton process whichever is the highest.

A further feature of one form of the invention resides in enclosing thereactor compartment in such a manner that back pressure in the vapour orgas space is used to control the rate of alchlor addition; preferablythe reactor compartment is totally enclosed to the gas phase, so that ifan excess of alchlor is fed it accumulates as a gas and builds uppressure within the compartment. If the alchlor supply to thiscompartment is only at low pressure, then the accumulated pressure inthe compartment retards or stops the flow of alchlor into thecompartment and this state continues until suflicient of the excessalchlor vapour is consumed by fresh reductant contained in the liquidmetal received from the electrolytic cell to reduce the back pressureand allow further flow. This is a very desirable feature as the alchlorfeed rate is thereby automatically controlled according to demand.

Whereas the normal method of alloy manufacture would be to add thealloying element directly to the metal stream in most instance, where analloying element forms a volatile chloride, the alloy may be produced byintroducing the chloride of the element in addition to alchlor into thereactor compartment at the appropriate rate.

The process of this invention has a number of advantages over existingprocesses for the electrolytic produc tion of aluminum. It consumesvirtually no carbon, continuously produces aluminum at high currentefliciencies and enables operation at temperatures above the meltingpoint of aluminum yet significantly below those used in current aluminumextractive technology. By the use of gaseous alchlor a high purity feedcan be introduced and a closed cell Construction used with severalattendant advantages.

Important advantages derive from the introduction of the gaseous alchlorin a separate compartment to the electrolysis, in that a wide selectionof cell electrolytes is possible and a composition can be used whichgives high conductivity and low specific gravity. Resulting from this,high current densities are possible, close electrode spacing can be usedand voltage drop across the electrolyte can be minimised. Fixed anodesof optimum design can be used with a low voltage drop across them.Similarly cathode connector design can be improved to reduce voltagedrop at this point. Cell designs are possible which are amenable toautomation so that routine labour and supervision requirements arereduced to a minimum.

Because aluminum compounds are not directly electrolysed, but instead ametal reductant is deposited as a solution in a flowing stream ofaluminum or aluminum alloy, problems such as cathode over-potential andfog formation are substantially reduced and high voltage and currentefficiences are possible. By recycling the aluminum at a sufficient rateto ensure that the solution of reductant in the molten cathode isrelatively diluent, the higher voltages required for decomposition ofthe reductant salt in the electrolyte rather than of alchlor are largelyoffset by the reduced activity of the reductant dissolved in the moltencathode.

We have determined that substantially all the aluminum presentOriginally in the molten salt mixture may be removed by the use of onlya small excess of metallic reductant. We have also determined that theuse of a small excess of aluminum chloride in a molten salt will removethe reductant metal from the metallic product.

Examples of the operation of the process of this invention are asfollows:

EXAMPLE 1 A fused salt electrolyte comprising 20% w./w. magnesiumchloride 40% w./w. lithium chloride and 40% w./w. sodum chloride waselectrolysed at 670-700 C. in an alumina-lined cell fitted with aVertical .carbon rod as anode and having a pool of commercial-gradealuminum as cathode. The interpolar gap was 0.71 inch, the currentdensity 1080 amps per square foot and the overall voltage 3.2 to 3.4volts.

After electrolysing for one hour the magnesium chloride content wasreplenished to 25% by adding additional molten magnesium chloride.Electrolyss was then continued for a further period of one hour afterwhich the aluminum metal product was removed and on analysis was foundto contain 397% w./w. metallic magnesium.

EXAMPLE 2 A reaction crucible was prepared by lining a mild steelcrucible with high alumina refractory material. The crucible was fittedwith a vertical shaft carrying an impeller at its lower end. 1,700 gms.of an aluminum-magnesium alloy, containing 65% magnesium, were added tothe crucible followed by 500 gms. of sodum chloride, the quantity ofalloy being selected to cause the interface between the metal and thesalt phase, in the molten state, to pass through the impeller.

The crucible and contents were placed in a steel retort, evacuated,filled with argon and then heated to 750 C. Pure gaseous aluminumchloride was then fed in at a controlled steady rate (1.5 litres perminute S.T.P.) to the atmosphere above the salt phase. At the same timethe impeller was rotated to splash molten metal onto the wall of thecrucible and thereby increase the contact between the molten metal andthe salt at the interface. The aluminum chloride was rapidly absorbed bythe sodum chloride to form a double salt sodum aluminum chloride) andthe contact between the double salt and the aluminum-magnesium alloy atthe interface resulted in a reaction between the double salt and themagnesium to yield magnesium chloride and to return sodum chloride tothe salt phase for absorption of further quantities of aluminum chlorde.

When a very slight excess over the stoichiometric amount of aluminumchloride had been added to the apparatus, feeding of the aluminumchloride was discontinued and the apparatus was allowed to cool. Theresulting ingot of aluminum was dense, free from chloride inclusions,and contained approximately 0.002% of magnesium.

In one method of Operating the process continuously the metal reductant,magnesium, is fed at a known rate into the upper section of a verticalreactor containing molten aluminum and fused sodium chloride. Gaseousaluminum chloride is fed under pressure into the upper part of thereactor at a rate stoichiometrically equivalent to that of themagnesium. Product metal is drawn off from the base of the reactor at acorresponding rate, whilst magnesum chloride overflows a weir at the topof the reactor and may be diverted to an electrolytic cell forregeneration of magnesium and chlorine.

To facilitate the Chemical reactions the contents of the reactor arecontinuously agitated.

To minimise the effect of fluctuations in the rate of flow of reactantsit is found convenient to maintain a reserve of magnesium in the mainmetallc phase, this section being recirculated from a point a shortdistance above the product tapping point. Makeup magnesium may be addedto the system as an alloy or pure metal may be blended into therecirculating stream.

We have found that aluminum chloride even in molten admixture withvarious other chlorides will react with sufficiently electro-positivemetals at high efficiency, producing molten aluminum containing if sodesired, only trace amounts of the metallc reductant.

We have demonstrated, however, that the position of equilibrium in thisreaction is such that substantially all the aluminum chloride presentOriginally in the molten salt miXture may be removed by the use of onlya small excess of metallc reductant. We have also` shown that the use ofa small excess of aluminum chloride in a molten salt will remove thereductant metal from the metallc product.

EXAMPLE 3 85 gms. of magnesium ingot in a fused alumina crucible (8" X3" diameter) were heated to 700 C. under an -argon atmosphere in a steelcrucible. Lumps of a double salt of aluminum and sodium chlorides (74.8%aluminum chloride) were added as required to maintain the temperature inthe range of 750 to 800 C. until a total of 592 gms. had been added,agitating continuously with a tantalum stirrer. After cooling, the solidsaline and metallc phases were separated mechanically and analysed. Themagnesium content of the metallc aluminum phase was less than 0.004%.The aluminum chloride content of the saline phase ranged from 16.6%

EXAMPLE 4 5275 gms. of an aluminum-magnesium alloy (90.4% magnesium)were heated to 700 C. as described in the Example 3; after which 289gms. of a double salt of sodiurn and aluminum chlorides (73.7% aluminumchloride) were added progressively with st-irring. After cooling andseparating the two phases, 517.5 gms. of aluminum Were obtained with amagnesium content of 0.004 to 0.04%; the 274 gms. of saline phasecontained 3.1 to 37% aluminum chloride.

The preceding Examples 3 and 4 demonstrate the effectiveness of thisinvention in the batchwise production of aluminum. There are advantagesattached to carrying out the process in a continuous manner. This canreadly be achieved, using general procedure of countercurrent flow insutably designed contactors.

The form of apparatus for carrying out the invention, which is showndiagramnatically in the accompanying drawings, will now be described.These drawings and the description thereof are illustrative only, andare in no way to be regarded as limitative of the invention. In thedrawings the same reference numerals are used to refer to like orcorresponding parts.

In the drawings: FIGURE 1 is a View in sectional elevation of apparatusfor the electrolytic production of aluminum;

FIGURE 2 is a sectional view taken on line 2-2 of FIGURE 1; and

FIGURE 3 is a sectional plan view taken on line 3--3 of FIGURE 1.

The apparatus ind-icated generally at 1 is provided with an outer 'shell2 which is thermally insulated by the lining 3. An inner graphite and/orcarbon lining 4 contains the contents of the apparatus and carriescurrent from the connectors 6 to the cathodic liquid metal layer 7. Thesides of the electrolysis compartment or cell 11 are electricallyinsulated by a refractory lining 5.

The apparatus 1 is divided internally by graphite and/ or carbon wall '8and by graphite carbon and/or refractory wall 9 into five compartments,narnely the electrolysis compartment 11, the pump compartment 12, thefirst stage reactor compartment or vessel 13, the second stage reactorcompartment or vessel 14 and the product metal storage sump 15. Theelectrolysis compartment 11 contains a molten salt electrolyte 16 whichis of lower specific gravity than the cathodic metal layer 7therebeneath. Anodes 17, 18 preferably of graphite or carbon depend intothe electrolyte 16 and are Secured to the cover 19, the anode connectionbeing indicated at 20. The undersurface of the cover 19 not protected bythe anodes 17, 18 is line'd with refractory block 21.

A chlorine gas outlet 22 is fitted through the cover 19 and block 21 andcommunicates with the space above the electrolyte 16. Direct current issupplied to the connectors 6 and 20. A cover 23 is fitted to the firstand second stage reactor compartments, the pump Compartment and theproduct metal storage sump. Refractory block linings 24 are used toprotect the undersurface of the cover 23. The covers 19 and 2.3 aresealed and gas tight. Refractory block lining 25 protects the walls ofthe reactor compartments 13, 14.

A pair of vertical shafts 26 driven by motors (not shown) projectthrough glan-ds 27 in the cover 23 and are fitted with refractory-cla-dirnpcllers 28 which have in clined blades adapted to spray upwardly theliquid metal 7 which enters the first and second stage reactorCompartments 113; 14 from the electrolysis compartment 11 throughpassageway 29 which is at such a level that the interface between thecathode metal layer 7 and the electrolyte 16 passes through it. Asimilar passageway 30 for cathode metal and electrolyte extends betweenthe electrolysis compartment 11 and the pump compartment 12.

A wall or bafie 31 is provided in the electrolysis compartment 11 andextends from the end of the said compart-ment which is nearest to pumpcompartment 12, to a point spaced a short distance from the opposite,end of the compartment 11. The wall or baflle 31 forms 'a trough 32between itself and the adjacent wall of the compartment 11. Thepassageway 30 communicates with one end of the trough 32. A wall or weir33 between pump compartment 12 and product metal storage sump 15controls the metal level throughout the system. A downwardly slopngpassageway 34 is provided in the bottom of sump 15 for periodic t-appingof product aluminum, the outlet from passageway 34 being normally closedby a removable plug 35.

A passageway 36, through which the cathode metal/ electrolyte interfacepasses, permits flow of electrolyte from the first stage reactorcompartment 13 to the second stage reactor compartment 14 and permitsflow of cathode metal in the opposite direction. The flow of met-al iscontrolled by a lip 37 which is located inside the second stage reactorcompar-tment 14 and rises above the cathode metal/electrolyte interface.Metal is delivered from the second stage to the first stage by beingsplashed against the dividing wall by impeller 28 and then running downinside lip 37 and through assagew-ay 36.

A metal outlet passage 38 is provided to permit flow of -metal from thefirst stage reactor compartment 13 to the pump compartment 12, thispass-age being located below the surface of the met-al. The pumpcompartment 12 is provided With -an electrolyte metering pump 39 of thegas displaeement type adapted to deliver electrolyte 16 entering pumpcompartment 12 through passage 30 at a controlled rate, when sorequired, through a passageway 40 to the first stage reactor compartment.13. Alchlor vapour is supplied through the main supply pipe 41 `andreractory sparger pipes 42 which project downw ardly through the cover23 into the first stage reactor compartment 13 terminating above thesurface of the electrolyte.

As illustrative of the process, the cell is filled with sufficientmolten aluminum or aluminum 'alloy 7 to cover the bottom of theelectrolysis compartment 11 and give good electrical contact with thelining 4. A metallic reductant, e.g. magnesium, is added and thensufficient molten electrolyte 16 is poured into the cell to cover thebottom of the anodes 17, 18. The molten electrolyte comprises an alkalimetal halide and may also include a selection of alkaline earth metalhalides and rare earth metal halides. Preferably it contains sodiumchloride.

The source of direct current is connected to the cell and electrolysisof the electrolyte 16 in the electrolysis compartment 11 takes placewith deposition of metal reductant, e.g. magnesium, in the c-athodiclayer 7, the metallic reducant entering into solution in the moltenaluminum. After a suitable period the electrolyte metering pump 39 andthe impellers 28 are started to circulate electrolyte clockwise throughthe cell as seen in FIGURE 3 -and to circulate molten metalanti-clockwise as seen in the same figure.

-When the molten metal is circul-ating through the first and secondstage reactor compartments 13, 14 alchlor vapour is fed from the main 41through the pipe 42 into the atmosphere above the electrolyte in thefirst stage reactor compartment 13. Pressure of alchlor in the main 41is maintained at aconst-ant small positive value. This forces the vapourinto the reactor chamber 13 until such time as the pressure built up inthe gas space of the chamber is sufiicient to halt the vapour flow. Whenthis pressure is relieved by further reaction of the alchlor vapour -inthe gas space with the electrolyte more alchlor will feed from the main41. The alchlor forms -a double salt with the electrolyte, e.g. if theelectrolyte is sodium chloride, sodium aluminum chloride is formed.

The electrolyte fiowing from the first stage reactor compartment 13 tothe second stage reactor compartment 14 through passageway 36 consistsof the double salt, e.g. sodium aluminum chloride, and a proportion ofthe chloride of the metallic reductant, e.g. magnesium chloride.

Molten metal comprising aluminum or aluminum .alloy and the metallicreductant, e.g. magnesium, enters the second stage reactor compartment14 from the electrolysis compartment 11 through passageway 29. In thiscompartment the reductant concentration in the aluminum is lowered by areaction between the reductant and the double salt. For example, sodiumaluminum chloride reacts with magnesium to yield sodium chloride andmagnesium chloride which are recycled through passageway 29 to theelectrolysis compartment 11 where the magnesium chloride iselectrolysed, the chlorine being discharged through outlet 22 and themagnesium going into solution in the aluminum in the cathode layer. Thesodium chloride is recycled to the first stage reactor compartment 13 bythe metering pump 39 where it absorbs alchlor as described above to formthe double salt sodium aluminum chloride.

The double salt reacts with the residual metallic reductant in themolten metal phase to yield the chloride of the metallic reductantdescribed 'above as comprising -a proporton of the electrolytecirculated from the first stage reactor compartment 13 to the secondstage reactor compartment 14.

Removal of the residual reductant enables substantially pure aluminum tobe circulated through passageway 38, pump compartment 12 and passageway30 to the electrolysis compartment 11. Excess aluminum fiows over weir33 into product metal storage sump 15 to be tapped off from time to timeby removal of plug 35.

What we claim is:

1. A process for the production of aluminum and aluminum alloys whichcomprises electrolysing in an electrolysis zone a fused salt electrolytecontaining at least one alkali metal halide, the cathode being moltenaluminum or aluminum alloy containing a metal reductant deposited fromthe electrolyte by electrolysis, transferring the molten aluminum oraluminum alloy and reductant to a reactor zone, reacting a double saltof alchlor with the metallic reductant in the reactor zone, transfer'ingthe chloride product of the metallic reductant to the electrolysis zone,re-cycling molten aluminum containing a lower concentration of themetallic reductant back to the electrolysis zone, and discharging themolten metal product from the reactor zone.

2. A process as claimed in claim 1 wheren alchlor as vapour is contactedin the reactor zone with a salt phase capable of absorbing alchlor as adouble salt.

3. A process for the production of aluminum and aluminum alloys whichcomprises electrolysing in an electrolysis zone a fused salt electrolytecontaining at least one alkali metal halide and at least one halideselected from the group consisting of the akaline earth metal halidesand the rare earth metal halides, the cathode being molten aluminum oraluminum alloy containing a metal reductant deposited from theelectrolyte by electrolysis; transferring the molten aluminum oraluminum alloy and reductant and electrolyte as a molten salt phase to areactor zone, reacting alchlor with the molten salt to yield a doublesalt and reacting the double salt with the metallic reductant in thereactor zone to yield the chloride product of the metallic reductant;transferring the chloride product of the metallic reductant to theelectrolysis zone, re-cycling molten aluminum containing a lowerconcentration of the metallic reductant back to the electrolysis zone,and discharging the molten metal product from the reactor zone.

4. A process according to claim 3 wheren alchlor is fed to the firststage of a two-stage reactor zone, the aluminum or aluminum alloy fromthe first stage being re-cycled to the electrolysis zone and the saltphase containing both the chloride product of the reductant and thedouble salt being fed to the second stage of said reactor zone to reactwith fresh molten aluminum or aluminum alloy and reductant from theelectrolysis zone.

5. A process as claimed in claim 4 wheren the salt phase containing thechloride product of the reductant and substantially denuded of alchlorin the second stage is re-cycled to the electrolysis zone.

6. A process for the production of aluminum and aluminum alloys whichcomprises electrolyzing a fused salt electrolyte containing at least onealkali metal halide, using as cathode a molten metal or alloy whichcontains in solution a metallic reductant deposited from theelectrolyte, and Contacting the molten metal or alloy and the containedmetallic reductant with a double salt of alchlor, the reaction betweenthe metallic reductant and the double salt of alchlor being eifected intwo stages, in which process the electrolyte and the molten metal aretransferred from the electrolysis zone to the reactor zone incounter-current flow, the alchlor being fed to the first stage of thereactor zone, the electrolyte being'fed initially to the first stage ofthe reactor zone and thence to the second stage of the reactor zone andthe molten metal being fed initially to the second stage of the reactorzone and thence to the first stage of the reactor zone.

7. A process for the production of aluminum and aluminum alloys whereina fused salt electrolyte containing at least one alkali metal halide iselectrolyzed using a cathode of a molten metal or alloy, in which theelectrolyte comprises a halide of a metallc reductant for aluminumchloride which reductant is soluble in the cathode, the metailicreductant being deposited by electrolysis from the electrolyte anddissolved in the cathode, and the cathode metal or alloy and thecontained reductant being brought into contact with a double salt ofalchlor.

8. A process according to claim 7, in which the molten salt electrolyteis electrolyzed in an electrolysis cell and the cathode metal or alloyand the contained metallc reductant are transferred'to a reactor inwhich a double salt of alchlor is contacted with the metallc reductantto produce a chloride of the reductant, which chloride is transferred tothe electrolysis ceil, the molten metal or alloy containing aluminum anda correspondingly lower concentration of metallic reductant, part ofwhich is fed back to the electrolysis cell and part of which isdischarged from the reactor as the molten metal product.

9. A process according to claim 7, in which the cathode comprises moltenaluminum or aluminum alloy.

10. A process according to claim 7, in which the electrolyte contains atleast one halide of a metal selected from the group consisting of thealkaline earth metals and the rare earth metals.

11. A process according to claim 7, in which alchlor is contacted with asalt phase which is able to absorb the alchlor to thereby form therewitha double salt.

12. A process as claimed in claim 11, in which the alchlor is at asuperatmospheric pressure in the reactor.

13. A process as claimed in claim 7, in which the metallic reductant ismagnesium.

14. A process as claimed in claim 7, in which the metallic reductant islithium.

15. A process according to claim 7, in which reaction between themetailic reductant and double salt of alchlor is carried out in areactor having two sections.

16. A process according to claim 15, in which alchlor is fed to thefirst section of the reactor and contacted with salt phase to form adouble salt of alchlor; and a portion of the salt phase and thecontained double salt is t'ansferred to the second section.

17. A process according to claim 15, in which cathode metal or alioy andthe contained metaliic reductant is transferred from the electrolysiscell to the second section and then to the first section of the reactorso as to contact the salt phase thereby to produce aluminum or aluminumalloy, a portion of which is transferred from the first section of thereactor to the electrolysis cell.

18. A process according to claim 17, in which the salt phase from thesecond stage of the reactor, is transferred to the electrolysis cell.

19. A process according to claim 7, in which the molten metal or alloyfrom the electrolysis cell and the double salt of alchlor are broughttogether in counter-current flow.

20. A process as claimed in claim 8, in which the nolten metal or alioyin the reactor is agitated.

References Cited UNITED STATES PATENTS 362,441 5/ 1887 Gratzel 204-67387,876 8/1888 Heroult 204-67 XR 2,742,418 4/1956 Padgitt 204--71 XR2,919,234 12/1959 Slatin 204-71 XR FOREIGN PATENTS 16,794 10/1889 GreatBritain. 757,908 9/1956 Great Britain.

JOHN H. MACK, Primary Examner D. R. VALENTINE, Assistant Examiner U.S,Cl. X.R.

